SINGLE-VORTEX-INDUCED VOLTAGE STEPS IN JOSEPHSON-JUNCTION ARRAYS

We have numerically and analytically studied (ac + dc)-driven Josephso n-junction arrays with a single vortex or with a single vortex-antivor tex pair present. We find single-vortex steps in the voltage versus cu rrent (I-V) characteristics of the array. They correspond microscopica lly to a single vortex phase-locked to move a fixed number of plaquett es per period of the ac driving current. In underdamped arrays we find vortex motion period doubling on the steps. We observe subharmonic st eps in both underdamped and overdamped arrays. We successfully compare these results with a phenomenological model of vortex motion with a n on-linear viscosity. The I-V characteristic of an array with a vortex- antivortex pair displays fractional voltage steps. A possible connecti on of these results to present-day experiments is also discussed.